The present invention relates to a novel inorganic filler and to a thermoplastic resin filled with it. More particularly, this invention relates to a powdery magnesium hydroxide having a sorbitan ester coating as a flame retardant filler for thermoplastic resins.
To overcome the poor affinity of hydrophilic inorganic fillers to oleophilic resins, the art has coated the surface of the fillers with fatty acids and metal salts thereof but these agents have low softening points and their covering power is seriously reduced at the high temperatures sometimes imposed on the resins in use. Takahashi, in U.S. Pat. No. 4,126,593, shows that various problems encountered with the fatty acid and metal salt coatings can be solved by using a fatty acid ester of a polyhydric glycol as the coating for the filler. One of the problems was that when inorganic hydroxides are used in an amount sufficient to impart flame retardancy, the flow properties of the filled resin are impaired so that the workability of the resin in molding and extruding operations suffers. Other problems often arising from the coating of the fillers with fatty acid esters are the dulling of the filler's patina, thence that of the filled resin, and an undesirable color imparted to the filler and to the resin.
By use of the sorbitan ester coated magnesium hydroxide of this invention in a thermoplastic resin, the fire retardancy of the resin is improved while retaining good practical moldability without imperfections in appearance such as the occurrence of flashes on the surface of the molded article and without a substantial deterioration of the resin's mechanical properties such as impact strength. The workability, gloss, and color of the filled, flame retarded resins of the present invention is improved. Furthermore, the weight of the coating may be reduced considerably in comparison with coatings of the prior art. This reduction in weight makes the flame retardant more economical. Smoke suppression, often required by regulatory bodies, is afforded by the coated magnesium hydroxide of this invention and the effect is enhanced somewhat by the lesser amount of organic material being added to the total.